PETROGENETIC SIGNIFICANCE OF SOLID CARBONATE INCLUSIONS IN APATITE OFTHE SUKULU CARBONATITE, UGANDA

Many solid inclusions occur in apatite of the Sukulu carbonatite, Ugan da, of which the most abundant are carbonate, which can be classified into clear (Mg-calcite) and pitted (calcite) inclusions based on their morphology, texture and chemical composition. Although such solid inc lusions are ubiquitous in carbonatite apatite and have been described by many workers, this study provides new insight into their genesis an d petrogenetic significance. The pitted inclusions commonly have elong ate or spherical shapes and are spatially related to microfractures in the apatite host. They probably developed from early primary aqueous or Mg-calcite solid inclusions by infiltration of post-magmatic fluids through the microfractures. The clear inclusions generally have spher oidal shapes and are thought to represent an early magmatic phase and to be typically magmatic in origin. Electron microprobe analysis indic ates that the clear inclusions contain > 1.6 wt.% MgO and the pitted o nes <0.5 wt.% MgO. The FeO content also differs, the clear inclusions containing about twice as much FeO as the pitted ones. Heating experim ents suggest that some of the clear inclusions must be representative of crystals precipitated at high temperature (>1100 degrees C) prior t o apatite formation. In contrast, other clear inclusions became dark o r brownish and remarkably homogeneous on heating at relatively moderat e temperatures (740-912 degrees C) indicating that they may represent true melt inclusions trapped as melts during apatite growth. The prese nt findings clearly illustrate the importance of both magmatic and pos t-magmatic processes in the genesis of the carbonate of the Sukulu car bonatite complex. They also suggest that extensive postmagmatic proces ses are likely to have been responsible for development of the low Mg- calcite and associated dolomite which dominate the sovites of this com plex.